This invention concerns all terrain vehicles (ATV""s) used for recreational and utility purposes. These vehicles are open topped with two large rear wheels having large tread tires powered to propel the vehicle over various terrain.
While providing effective traction in most conditions, wheeled propulsion has its limitations, such as are encountered in snow and mud. Endless tracks have been used for many years in various types of vehicles intended for off road travel, and provide more effective drive in snow and other difficult terrain conditions. For example, recreational snowmobiles use endless tracks for propulsion in snow. Snowmobiles lack the maneuverability of ATV""s and the slider mounting used in snowmobiles relies on the presence of snow to reduce the high friction otherwise developed in the slider guide.
There have been prior attempts to convert ATVs to a track drive so as to allow their use in snowy conditions, and thus to provide an all season vehicle.
This conversion has in the past been attempted by installing endless tracks so as to be driven by the rear wheel drive. In the past, this has been done by installing sprockets on the rear wheel axle replacing the rear wheels, as shown in U.S. Pat. No. 6,095,275. The track band is circulated around idler wheels spaced to the rear from the sprockets. See also U.S. Pat. No. 4,699,229. The tracks must have a linear footprint as they circulate in order to produce an extended ground contact developing the improved traction enabled by use of the endless track drive.
A slider guide has been used in such conversions to guide and tension each track band, similar to that in use on conventional snowmobiles. As noted above, there is a high friction loss created in the slide assembly when not used in snow, which reduces the horsepower available to drive the track bands, adversely affecting the performance of the vehicle. The use of heavy sprockets replacing the wheels and tires adds to the time and cost of carrying out the conversion.
U.S. Pat. No. 4,635,740 shows an ATV conversion in which the tracks are directly driven by the rear tires, but such engagement is not effective at high speeds and is subject to slippage in snow or mud. An auxiliary set of wheels and tires is also used in lieu of the slider assembly, but this is a costly alternative.
It is an object of the present invention to provide a low cost ATV track conversion which maximizes the performance of the vehicle by providing a track drive and support which is efficient in not generating large frictional losses even when not operated in snow, and which drives, guides and supports the track bands effectively even at high speeds.
The above object and others which will be understood upon a reading of the following specification and claims are achieved by a conversion having several unique features.
First, the drive to a pair of endless tracks includes a lightweight molded plastic sprocket ring mounted to each existing rear tire of the ATV by being slipped over a partially deflated tire, with the tire then reinflated to be expanded to tightly engage the inside of the sprocket ring to be held in place on the tire very securely. The inside of each sprocket ring has pocket features interfit with the tire tread raised areas to establish a positive drive between each tire and the associated sprocket ring.
The exterior of each sprocket ring is formed with features engaged with corresponding raised engagement features on the inside of the associated endless track band.
According to a second unique feature of the present invention, each endless track band passes around one of a pair of idler wheel assemblies supported on an auxiliary axle housing mounted to the rear of the ATV drive axle by a swing arm extension pivotally attached to the end of the ATV swing arm which mounts the rear wheel drive axle.
Each idler wheel assembly includes an angled swing link pivoted on a stub shaft projecting from a respective end of the auxiliary axle housing and coupled to a torsion spring in the auxiliary axle assembly. The idler wheel assembly includes a pair of sets of return idler wheels mounted on one arm of the angled link around which the associated endless track band is circulated to define the rearmost extent of the track band, and passing around the associated sprocket ring at the forward end of the track band.
Two sets of track shape defining wheels are also included in each idler wheel assembly, a lower set supported on the other arm of the link forward of the return idler wheel set and engaging the inside of the lower segment of the track band. The other track shaping defining wheel set is supported on a pivoted link spring urged to swing up from the angled link so that the second set of track shape defining wheels engages the inside of the upper, return segment of the associated track band.
The swing arm extension itself has a spring-shock absorber unit attached at one end thereto, connected to the ATV swing arm to create an independent suspension for the idler wheel assemblies which augments the main ATV suspension, normally including the swing arm and an ATV spring and shock absorber unit. The provision of an auxiliary track-idler wheel suspension is a third unique feature of the track conversion according to the invention, allowing the track to be held in proper contact with the ground even at high speeds.